Wetlands may include marshes, bogs, and swamps. Wetland delineations tend to be controversial because such a determination often pits the interest of environmentalists against the interests of landowners and/or land developers. Therefore, standards or guidelines have been created to standardize wetland delineations. These guidelines also attempt to balance the interests of the public and private landowner/developer. According to typical guidelines, whether a particular parcel of land qualifies as a wetland generally depends upon the percentage of the growing season that the surface of the soil is continuously saturated with water.
One example of wetland delineation guidelines includes the Corps of Engineers' Wetlands Delineation Manual of January 1987 (“'87 Manual”). The '87 Manual provides guidelines that may be used to determine whether a particular parcel of land is a wetland. Generally, land qualifies as a wetland if it is continuously saturated to the surface between 5% and 12.5% of the growing season. However, the '87 Manual indicates that many sites are not wetlands despite being continuously saturated to the surface between 5% and 12.5% of the growing season. Delineation in these cases is left to the judgment of the delineator (person who determines whether a site is a wetland). According to the '87 Manual, sites not continuously saturated at least 5% of the growing season are not wet enough to be considered wetlands.
The '87 Manual provides that delineators may consider three parameters, soil characteristics, vegetation, and hydrology, when evaluating whether a site is a wetland. Soil characteristics may be used to determine whether the soils at the site are hydric soils. Hydric soils form under conditions of saturation, including flooding that persists long enough to develop anaerobic conditions in the soil. These anaerobic conditions, characteristic of hydric soils, may be observed as color changes in the soils.
Direct determination of the hydrological parameter is difficult. Capillary forces pull the water table down below the level of soil saturation. The '87 Manual defines wetland according to variations in “depth to saturated soils” during the growing season. Depth to saturated soils can only be measured directly when the effect of capillary forces are momentarily cancelled. This happens at the onset of rain or when an artificial stimulus is provided.
Wetland determination becomes controversial when a delineator relies mainly on vegetation and soil characteristics to assess how the hydraulic parameter depth to saturated soils varies during the growing season. Accurately evaluating the site according to its hydrology requires numerous visits. However, it is not uncommon for a delineator to make a determination based on only a single visit.
Digging a pit in the ground and measuring the depth at which water appears in the pit will yield the depth of the water table (the upper boundary of a free groundwater body at atmospheric pressure). However, the depth of the upper boundary of saturated soil (referred to hereafter as the depth to the saturated soil) is not necessarily equal to the depth of the water table. Capillary action may draw water up through the grains of soil to a level above the water table causing saturated soil to occur above the water table. (See FIG. 1A.) The volume of water located between the depth to the saturated soil and the water table is known as the capillary fringe. Both the depth to the saturated soil and the water table may rise during rainfall and shrink when depletion mechanisms such as drainage, evaporation, and transpiration deplete water from the soil.
The capillary fringe is, in essence, a zone of negative pressure that extends from the saturation level down to the water table. As shown in FIG. 1A, the pressure differentials across membrane spans, such as “a” and “b,” balance each other out. The spans are taut, there is no flow, and there is a negative pressure zone. The mechanics of capillary actions that creates the capillary fringe are discussed in applicant's corresponding provisional patent application, No. 61/668,322, incorporated herein by reference.
The following also provides an explanation of the mechanics of capillary actions that applicant believes is applicable to the creation of the capillary fringe:
1. Introduction to Capillary Mechanics
The reverse-Wieringermeer effect, reported by Hooghoudt (Waarnemingen van grondwaterstand voor de landbouw, Commissie voor Hydrologisch TNO, Verslagen Technische. Bijeenkomsten 1-6; 185-201 (1947)), is an enigma, tensile strength in water counter-intuitive, and the concept of “surface tension” does not help in the investigation of either one. The mysteries nest within each other like Matryoshka dolls. To explain the reverse-Wieringermeer effect, it is necessary to “visualize” the capillary mechanism, which in turn requires understanding that water has tensile strength. Also, some simplifying assumptions are required.
Stensrud (Simen Stensrud, lecture (Ila Elementary School)), ca. 1944) visualized that water molecules all “hold hands” with their neighbors, but those at the surface have no one on the outside to hold hands with so they double up on the grip with their extra hands, which causes “surface tension.”
As the scale is reduced from macroscopic to microscopic, “mechanics-as-we-know-it” “warps” to “capillary mechanics.” The generally accepted mathematics (ref. for instance, Dingman (S. Lawrence Dingman: Physical Hydrology, 2nd ed., Prentice Hall)), (eq. B5) does not warp along with the real world as the former contains a singularity. ‘Surface tension’ is misleading; there is tension everywhere in a body of water at the same time that there is compression, i.e. water is pre-stressed—and not just at the surface. “Tension” in water must reach the tensile strength before a molecule can be pulled away. A number of pitfalls are bypassed by not looking directly at the tension imposed by adhesive forces near a line of contact, but at the remainder when it is subtracted from the pre-tension that exist everywhere in liquid water.
The “stand-in” water molecule is made up from subassemblies, which are in turn made up from sub-subassemblies, etc., in the manner of “real-world” molecules. The polarity, and other inconvenient characteristics not important for visualization, can be “stripped away” from the actual molecule.
Attractive forces pull molecules toward each other until they are one “diameter” apart. A growing repulsion keeps them from getting closer to each other. The molecules “levitate” off one another in a “no-touch” situation similar to when centrifugal forces “levitate” the Moon off the Earth. The molecules “cohere” and pre-tensions water; it is in tension and compression at the same time, and the pre-tension must be overcome before a molecule can be separated from a body of water.
In liquid water, molecules slip around each other with ease and reduce the potential energy of the attractive forces. An unconfined body of water assumes a spherical shape where the energy is least. If a “body of water” consists of three molecules, all touching each other, the energy per molecule required to rip one loose from another and swing it around and line them up in a row is relatively large. The energy requirement per molecule, in order to make and break bonds and move them around without stripping them away from the main body becomes progressively less in larger and larger bodies.
Cohesion dominates the “sphere-forming effort” in drops; however, it is a local phenomenon, and as a body of water grows in size, it “loses firmness.” A golf-ball sized body can be generated with some difficulty in a “gravitation-free” environment, but the cohesive forces can barely hold it together. In contrast to gravitational forces, cohesive forces do not ‘accumulate’ to a noticeable extent.
Gravity attracts everything to everything else but the force is so weak that it does nothing to hold small bodies of water together. A cumulative effect only becomes prominent in large bodies of water.
The attractive forces between glass and water are stronger than between water and water. Water “adheres” to glass but if the adhesive forces equaled the intermolecular forces, the angle of contact would be 90-degrees instead of zero.
Raindrops and planet-size bodies of water are free (self-contained) and ‘separate’ when no external forces interfere with cohesion and gravity. But soil particles, which are assumed to be made of glass, partially confine water in the interstices. At the air-water interface, adhesive forces at the lines of contact drag water along the confining solids.
Tight spaces “warps mechanics into capillary mechanics” and gives rise to the reverse-Wieringermeer effect—which Einstein (Albert Einstein: Folerungen aus den Capillaritätserscheinungen, Annalen der Physik, 4 (513-523)) had not heard of when he published his first paper in 1901. Einstein brought other substances than glass, water, and air to the table.
2. Pre-Stresses in Liquid Water
If “theoretical tweezers” lift one water molecule at the surface in a glass of water up, other molecules will hang on below due to cohesion. In turn, more and more hang on until the weight of water snaps the bonds at the upper molecule where water reaches its tensile strength.
Because molecules have dimensions, the lifting force on the upper molecule, represented by an arrow, must be the resultant of a distributed force. The cohesive forces that bond the upper molecule to the next ones down are also resultants of distributed forces. The arrows in a typical vector diagram do not reflect this, which means that the theoretical stress, in terms of force per unit area, is infinite and the mathematics reflects a singularity. An improved model of how forces work on matter at the molecular level is needed so that equations can be written to apply to this situation.
Before water molecules can be separated from one another, they slip around and the force to lift one molecule from a surface spreads out over more and more molecules and the stress at the ‘hangers-on’ diminishes rapidly.
In a glass of water, a “meniscus” is visible at the wall. The stress at the line of contact, where the meniscus surface intersects the wall, must equal the “bulk tensile strength of water” if the angle of contact is zero. For a mathematical surface, where the stress is zero, the water table, is not far below.
To visualize “tension” in water, it is best to look at what remains after it has been subtracted from the pre-tension that exists everywhere in liquid water. At the line of contact, it is zero and at the water table, pre-tension remains. The force-field lines of this “differential tension” converge and meet at the line of contact where the ‘tension’ reaches the “tensile strength” of water, which means that the molecules are ripped away from one another.
By subtracting “tension” from pre-tension, the “uncertainties” at the line of contact are stripped away, but only there. Near the line of contact, but not at it, the two forces are not equal. To what extent they grow different further away is uncertain except it does not matter for purposes of this analysis. There are uncertainties regarding how “Fermions” (matter particles) and ‘Bosons’ (force particles) interact at the molecular level but the problem belongs to the physicists and rather than to consult with them, one uncertainty is subtracted from another and it is assumed that they are equal at the line of contact. That makes the differential zero at the line of contact. With that, the math challenge becomes trivial.
All uncertainties are lumped into one by saying that somewhere near the line of contact the “tension” equals the “bulk tensile strength” of water. In order to explain the reverse-Wieringermeer effect, it is not necessary to know about the “polarity” of water molecules, and so on. In a capillary tube, the up-forces near the line of contact divided by the cross sectional area of the tube yields stress; it is a negative pressure that lifts water up to a height that is inversely proportional to the tube diameter.
It is “inversely proportional” if working with “surface tension” as well, but then one may lose sight of the tension that exists elsewhere. The spacing between the equipotential lines of the “remainder” after tension is subtracted from pre-tension, is zero at the line of contact, and widen towards the water table where the tension is equal to the pre-tension in the water.
3. The Capillary Effect
The geometry of the air-water interface curvature resembles an “Euler spiral” more than a meniscus; the tension in the water is high where the radius of the curvature is small and approaches zero if and when the interface “flattens out.” In a glass of water, the “flat portion” of the interface coincides with the water table. But where the air-water interface begins to curve upward, the water table extends to the wall. Whether the water table is entirely flat or not is not of consequence here.
A capillary tube has a small diameter and the curvature of the air-water interface does not “flatten out,” it “turns around and goes back up on the other side.” Instead of “petering out” towards the water table, the tension only dilutes from the line of contact to the nadir of the curvature where it is still negative. Confined by glass, the tension in the water continues down the tube from the nadir of the air-water interface to the water table. Capillary action results when the forces at a line of contact do not dilute entirely before the air-water interface ‘turns around and goes back up the other side’ due to the confinement of a “capillary space.”
It is not necessary to know the exact tensile strength of water in order to calculate that the rise of water in a capillary tube is inversely proportional to the diameter. In that case, it is necessary to know the “diameter” of the molecules and how the ‘tension’ varies across their “diameters”. However, exact values are not needed as long as the analysis is limited to saying that the ‘remainder-tension’ is zero at the line of contact and at the ‘pre-tension level’ at the water table.
4. Separate Bodies of Water
Liquid water only exists as “separate bodies,” they can be confined, partially confined, unconfined—i.e. self-contained. A self-contained body of water assumes a spherical shape when no external forces work on it. If water is confined, the pressure goes up everywhere if a single molecule is added; the “communication” within one and the same body is flawless.
No stress information is exchanged between the seven seas and a small drop of water hovering just above the surface because they are separate bodies. Drops forming at a leaking faucet are part of a body of water that extends down through the plumbing from “a lake somewhere,” until they gain weight and snap off. If a faucet is carefully opened so there is a glass-smooth jet coming out, it is possible to see how cohesive forces fight gravity. The “specimen” necks down as gravity accelerates it while cohesive forces “grow drops.” They break loose and momentarily the “tip of the jet” is “pointy,” but new drops soon form.
5. The Reverse-Wieringermeer Effect
Water table measurements taken over a nine month time period (1997-1998) by the applicant, about 4000 in all, showed that the water table can move up much farther and faster at the onset of rain than expected from the water input. An explanation of the phenomenon is provided in an article by Gillham (The Capillary Fringe and its Effect on Water-Table Response, Journal of Hydrology, 67 (1984) 307-324 S.B.). Gillham found that the application of 3 mm of water to the soil surface on a test site shifted the water table about 300 mm in less than 15 seconds. Hooghoudt (see Section 1 above) had noted this (1947) and labeled it the “reverse-Wieringermeer effect.”
6. Spans
Air-water interfaces stretch or “span” between lines of contact. If the spans are short, like in capillary tubes, stress concentrations in the water near the lines of contact do not fully dissipate by mid-span; a negative stress or pressure continues from there down the tube until it reaches zero at the water table where it turns positive. As spans get shorter, stresses increase in inverse proportion to span length.
7. Water Between Marbles
A small amount of water between two marbles will stay tenaciously in place when the marbles are moved around. The water necks down and snaps like a “test specimen” when the marbles are pulled apart. There are two lines of contact and a single span stretching between them.
The water bonds marbles, although with “shooting” size ones the “bonding forces,” are hardly noticeable. If a number of marbles are assembled and bonded with small amounts of water, the “construction” has little “structural strength.” However, the negative pressure at each span is inversely proportional to its length, and the “tensile strength” of the whole “structure” ends up increasing in inverse proportion to the marble diameters.
High stresses, or negative pressures, reduce evaporation rates. If spans are short, water may not evaporate. Aside from a molecular layer of water adhering to them, soil particles tend to be dry between lines of contact. The stresses in partially-confined water in soil-particle interstices are not affected by this.
8. Scale-Related Warp
If three (vertical) glass rods are clamped together like a bundle of straws with one end stuck in a dish of water, they constitute a “tube” (with a non-circular hole) and a top view of the construction will look like “three coins, all touching, laid out on the table.” If the rod diameters are as large as those of pennies, there will be no visible “capillary action” except small menisci near the lines of contact.
If the scale is reduced, water will start to rise between the rods. Adhesive and cohesive forces overcome gravity in “capillary” spaces as the scale goes from “macro” to “microscopic”.
At small scales, the rods can even be spaced a “capillary distance” apart. If the “coins-on-the-table” are spaced a “coin-thickness” apart they form a “leaky” tube. However, a cross section taken half-way up the column of water in small-scale “leaky tubes,” shows that instead of leaking out the water stays in place. The three convex air-water surface spans reflect a stress in the water behind them that controls their movement, while the spans are biased to move outward, they lose “strength” relative to another as they move because other spans must shorten when the volume of water is constant.
From top to bottom, the air-water-interface spans in the leaky tube are as short as they can be near the top, where they draw the highest negative pressure in the water. Toward the bottom, the spans get longer until there is no negative pressure behind them at the water table. There, the effort of gravity equals the combined efforts of adhesion and cohesion; the resulting stress or pressure is zero, and the water “leaks” out.
A paper towel is a “leaky” structure that “wicks” water up to a height of maybe 10-inches. The pressure is, maybe, negative 10-inch of water at the top and zero at the water table.
9. Water Between Two Flat Plates
In order to determine the “bulk tensile-strength of water,” consider two circular, flat plates. Place a small amount of water between them. Keep the plates parallel and centered while pulling them apart. The negative pressure in the water varies in inverse proportion to the span length—the distance between the plates.
With flat plates, little water and short spans, the stress is as high as it can get. The stress at the nadir of the air-water interface curvature continues throughout the body of water and we can calculate it at the time the bonds break. This would be close to the “bulk tensile strength of water” while the “theoretical stresses” near a line of contact would be higher.
10. Air-Water Interfaces in Soils, Fronts
Consider the air-water interface in a bucket of potatoes half filled with water. It resembles a perforated plate with part of a potato stuck in each hole, or perhaps the map of a lake with islands, some poking out of the water from below and others dipping into the water from above. The outlines of the islands are “lines of contact”.
Next, visualize soil as small glass particles of random size and shape. Forces at the lines of contact will try to move the spans in the “perforated plate”—the air-water interface—until the pressure mid-span everywhere is the same. The spans sag like “hammocks” in reaction to negative pressures between lines of contact as the air-water interface adjusts in order to equalize pressure.
The spans between the particles lengthen and shorten as the air-water interface propels itself down the interstices. A front at the air-water interface seeks an equilibrium position where any span that lengthens when it tries to move forward will be held back by spans that shorten in response.
Air-water interfaces in soils are “fronts,” some are “open” and others “closed.” A closed front encloses a body of water while an open front does not. Opposing spans are mostly responsible for negative pressures in enclosed bodies while the weight of water determines the negative pressure below open fronts.
11. Moisture
The water within a closed front is a separate body of water. Moisture can be regarded as small bodies of water within closed fronts. As a front moves toward an equilibrium position, the water may subdivide. Evaporation reduces both volumes and pressures of bodies of water. The pressure may go so low that plants cannot access it.
12. Ground Water
Ground water is commonly considered to be water below the water table, i.e. water under positive pressure. However, open fronts at the air-water interface pull the water table down in dry weather when there is no water input. Meanwhile, the saturation level stays where it is. Lest there be two ground-water levels, it is considered that the level revealed by the water table at the onset of rain is the “proper” one.
The negative pressure at a front pulls the water table down, creating a negative-pressure zone or capillary fringe. However, a front is refractory; the first drop of water to merge with it at the onset of rain cancels the negative pressure near the point of intrusion. The water table moves up quickly, 300 mm in 15 seconds as Gillham (5) measured it. That is too quickly to consider flow; the water is already there and the water table merely shifts within it.
As more water follows, the situation becomes chaotic and remains so until the water input stops so that the air-water interface spans can “close ranks” and pull the water table back down. Data from 1997-1998 indicate that it takes longer to re-establish a capillary fringe than the 15 seconds or so that is needed to destroy it.
13. Miscellaneous Water-Related Phenomena
While the air-water interface in a capillary tube is enclosed within a circle; in a soil it is excluded from areas that lie within lines of contact. The propulsive power is still proportional to the cosine of the angle of contact except the surface geometry is no longer that of a cylinder but of a variety of objects, including soil grains, rocks, roots, and voids.
Whether a front exists in a tube or in a soil, span length is essential. In a tube, the air-water interface at the front spans the length of the diameter. In a soil, there are many spans of various lengths that keep adjusting as the front moves toward a position and the pressure behind all find a position where the forces balance. The spans sag under stress, and the negative pressures must be the same at mid-span everywhere when the elevation above the water table is counted in and the water doesn't flow.
Adhesive forces draw water towards the void in pits, like those dug to investigate water levels, but they run out of soil grains to latch onto before they get there. A front stops as it nears the wall, dropping from the saturation level, to the water table, where it crosses over before rising back up along the opposite wall. No visible “ring” reveals “depth to saturated soils”, as the '87-Manual (U.S. Army Corps of Engineers: Wetlands Delineation Manual, Technical Report Y-87-1 (January 1987)) calls it. The capillary fringe hangs with its whole weight from soil particles at the saturation level, and the reaction forces from the weight of water transmit downward and “compress” soil particles.
Sand consists of relatively large particles. Water under negative pressure lends some structural strength but that diminishes as it evaporates. Sand castles collapse when they dry out. Saturating water under positive pressure pushes the walls into the pits when digging for clams and it pushes the lower parts of sand banks out while negative pressures higher up lend structural stability there. The angle of repose varies with water content.
Soils have smaller particle sizes than sand, and water that remains locked up in lumps of dry soils gives them structural strength until they are crushed. The water quickly evaporates and there should be a small, but measurable, weight difference between a lump and the dust that is left after it has been crushed.
Particle size and shape play a role in determining whether or not large closed fronts exist in clays. “Quick clays” are saturated with water within closed fronts where short spans keep it from evaporating. Reaction forces clamp the particles together as if the air was pumped out of a rubber bag full of sand. Momentarily disturbing the front with a hard blow cancels the low pressure within. Visible water, liquefaction, may show up in pottery clays and landslides.
A piece of paper placed on top of potatoes in a bucket contacts them at points. The air-water interface of a drop of water that rests on a dry soil deforms so that the lines of contact will not be at points but they enclose “areas.” While the air-water interface in a capillary tube is enclosed within a circle; the air-water interface of a drop has areas that are excluded from it, like the holes in a perforated plate. Forces at the lines of contact that outline the excluded areas, assisted by the weight of water, draw water toward the interstices while cohesive forces try to keep the water from deforming. Unless there is sufficient moisture present, the pressure at the contact area due to gravity is too small to overcome the cohesive forces and the water “beads up” on top.
14. Terminology                Capillary Fringe: Water under negative pressure between the saturation level and the water table.        Front: The air-water interface (surface) in a soil; capillary forces bias it to move.        Depth to Saturated Soils: The vertical distance from the surface to the saturation level.        Ground Water: Water below the water table, water under positive pressure.        Moisture: Isolated bodies of water in the soil, generally above the saturation level.        Negative-Pressure Zone: The capillary fringe.        Reverse-Wieringermeer Effect: The rapid shift of the water table to the saturation level at the onset of rain.        Saturated Soils: A soil is saturated when the interstices between soil grains are completely filled with water.        Saturation Level The level below which a soil is saturated.        Single Spans: Bodies of water lodged between two glass beads or marbles have single spans.        Span: The distance between lines of contact at the air-water interface.        Water Table: A surface at atmospheric pressure. It shows up physically in a dug hole or a well.        
The capillary fringe makes it difficult to accurately determine the saturation level. The present application provides systems and methods for temporarily eliminating the negative pressure zone by disturbing the existing pressure differential at the saturation level. As a consequence, while the capillary fringe is temporarily collapsed, the water table moves up to the saturation level. This is illustrated in FIG. 1B.